Here are images of my Dahon Mu XL when I took the headset apart to grease the bearings.
1) Remove the center bolt (this provides pressure on the headset bearings when the handlepost is reinstalled)
2) Loosen the side bolt and remove the handlepost from the steerer tube.
3) At this point you should be able to slide the steerer tube out of the headset bearings.

Replacement of the fork is the reverse. You will need to tighten the center bolt to provide proper headset adjustment before tightening the side bolt. You should be able to find the correct torque settings on the Dahon site.

I agree with those ^^ who recommend against removing the fork if it's not absolutely necessary.
Steve